The Gemara (Taanis 10a) concludes that in Eretz Yisroel one begins reciting ve’sein tal umatar on the Seventh of MarCheshvan, whereas in Bavel one begins reciting it on the sixtieth day after the autumnal equinox.

[...]

This practice is followed in all of chutz la’aretz, even in places where rain is not seasonal, or where rain is needed earlier — although the precise reason why all of chutz la’aretz follows the practice of Bavel is uncertain (see Rashi and Rosh to Taanis 10a; Shu”t Rosh 4:10; Tur and Shulchan Aruch Orach Chayim 117).

He then goes on to cite some opinions that say that the date would vary based on national conditions, but says that we do not follow such opinions (see there for lots more - this is an oversimplification).